The invention relates to an adaptive method of encoding and decoding a series of pictures by transformation, and devices for implementing this method. The object of such a method is to reduce the quantity of information to be transmitted, or to be stored, when pictures have been digitized. It is applicable, for example, to digital video transmission systems or to digital video recorders.
It is known to encode a digitized picture by using a two-dimensional transformation of the cosine or Fourier, or Hadamard, or Haar, or Karhunen-Loeve type. Such an encoding consists in: dividing each picture into blocks of picture elements, each picture element being represented by a digital value which is the value of its brightness or of a colour difference: applying the transformation to each block in order to obtain a matrix of values called the transformation coefficients of the block; and in transmitting these transformation coefficients in an encoded form, for example using a Huffman code. The decoding then consists in: decoding the Huffmann code words in order to obtain the transformation coefficients; then in restoring the digital values representing each picture element by applying, to the transformation coefficients corresponding to a block of picture elements, the two-dimensional transformation which is the inverse of that used for the encoding. The transformations used in practice are transformations for which there exists fast algorithms, for example the cosine transformation.
The French Patent Application 2,575,351 describes an adaptive method of encoding and decoding consisting in:
dividing each picture into blocks of picture elements;
applying the cosine transformation to each block, the latter being represented by a block of brightness values in order to obtain a block of transformation coefficient;
determining, for each block, if it represents a scene with much movement or little movement;
transmitting the value of the transformation coefficients of the block if the latter represents a scene with much movement, or transmitting the differences in the value of these coefficients with respect to the coefficients of the similar block in the previous picture, if the block represents a scene with little movement;
transmitting an information indicating the type of encoding used for each block, these two types of encoding being called respectively intra-picture encoding and inter-picture encoding. The coefficients or differences of coefficients are transmitted in the form of Huffmann code words.
According to this known method, the decoding consists, before applying the inverse transformation, in determining a value of the transformation coefficients of each block representing a scene with little movement, by adding the difference in value of each of its coefficients respectively to the value of the coefficients of the similar block in the previous picture.
According to this known method, the encoding furthermore consists in applying a weighting to the values of the coefficients or to the values of the differences of coefficients, with a greater weight for the coefficients or the differences of coefficients corresponding to the low spatial frequencies of the picture, with respect to the coefficients or to the differences of the coefficients corresponding to the high spatial frequencies of the picture; and in quantifying according to a linear scale the weighted coefficients and differences of coefficients. The quantification step is variable according to the quantity of information to be transmitted. This is equivalent to multiplying all of the coefficients or all the differences of transformation coefficients of a block by a same coefficient called the quantification coefficient which is variable according to the quantity of information to be transmitted for the blocks of picture elements encoded before the block concerned; and in retaining only the whole part of the result of the multiplication.
The information to be transmitted is stored in a buffer memory enabling a transmission at a constant rate. A regulating device supplies a value of the quantification coefficient which continuously diminishes while the buffer memory is filling and which continuously increases while the buffer memory is emptying.
Naturally, the decoding furthermore consists in multiplying each transmitted coefficient value or each transmitted difference of coefficients value, by a coefficient equal to the inverse of the weighting coefficient used for the encoding; and then in multiplying it by a coefficient equal to the inverse of the quantification coefficient used for the encoding.
When a series of pictures represents a scene containing much movement, the quantity of information to be transmitted is high, and consequently the quantification coefficient is small in order to reduce the amplitude of the values of the transformation coefficients or differences of transformation coefficients to be transmitted. Furthermore, the weighting coefficients give greater weight to the transformation coefficients corresponding to the low spatial frequencies of the picture in order to transmit the essential information of the picture while sacrificing less essential information which corresponds to the high spatial frequencies of the picture.
When the series of pictures represents a scene with little movement or a static scene, the encoding of each block is of the inter-picture type in order to exploit the correlation existing between these successive pictures. From one picture to the next, the values of the differences of transformation coefficients of similar blocks have a decreasing amplitude and the quantity of information to be transmitted tends to reduce. The regulation then reacts by increasing the quantification coefficient. On the other hand, the information remaining to be transmitted no longer relates to the low spatial frequencies of the picture as it has been favoured by the weighting and has therefore been transmitted. The information remaining to be transmitted relates only to the high spatial frequencies of the picture and the latter are then transmitted with a large amount of information. After a time interval corresponding to several pictures, the totality of the information representing a static scene is then transmitted and enables the reconstruction of the scene with very good fidelity.
For the encoding and decoding of colour television pictures, the previously mentioned document suggests processing in parallel three series of digital values corresponding to a brightness signal and to two colour difference signals respectively.
This known method has two disadvantages: the fact of the parallel processing of these three series of digital values leads to the use of three buffer memories which must restore the encoded information with data rates having constant ratios because the transmission channel has a constant data rate. Now, the information data rates corresponding to a brightness signal and to two colour difference signals have extremely variable ratios because the saturation of the colours is very variable and can even be zero in the case of pictures containing only whites, greys and blacks. The fact of imposing a constant ratio between these three information data rates leads in practice to uselessly increasing the quantity of information transmitted, or in sacrificing a portion of the information corresponding to colour differences, which is harmful to the fidelity of the reproduction.
Another disadvantage results from the regulation used in this method. When, in a same picture, there is a succession of blocks encoded by an inter-picture encoding, the quantity of information to be transmitted being small, the regulation reacts by increasing the quantification coefficient and tends to maintain a filling of the buffer memory. If an isolated block, or several blocks are to then be encoded by an intra-picture encoding, because they correspond to a limited area which is in motion, it is suddenly necessary to transmit large amounts of information. The buffer memory being maintained practically full, the regulation can only react by sacrificing a large portion of the information to be transmitted, i.e. by suddenly reducing the quantification coefficient when the buffer memory approaches saturation. In such a case, the blocks of picture elements encoded by the inter-picture encoding are restored with excellent fidelity while the adjacent blocks, encoded by an intra-picture encoding are restored with mediocre fidelity. The difference in quality is then very noticeable because these two types of blocks are adjacent in the same picture.
The purpose of the invention is to overcome these two disadvantages of the known method. The object of the invention is an adaptive method of encoding consisting in particular in storing in a same buffer memory the information to be transmitted corresponding to the values of brightness and to the values of the two colour difference signals, and consisting in using weighting coefficients and identical quantification coefficients, except for the application of a constant, for the transformation coefficients or the differences of transformation coefficients corresponding to these three types of signals.
According to another feature, the method according to the invention consists in using weighting coefficients which, in addition to giving a greater weight to information corresponding to the low spatial frequencies of the picture, are also variable according to the quantity of information to be transmitted, in order to further reduce the weight given to the information corresponding to the high spatial frequencies of the picture when the filling of the buffer memory increases and approaches the maximum.
According to another feature of the method according to the invention, the quantification coefficient is variable as a function of the filling rate of the buffer memory, but with a discontinuity corresponding to a fixed filling threshold, in order to be constant below this filling threshold and in order to increase when the filling rises above this threshold.
According to the invention, an adaptive method of encoding and decoding of a series of pictures by transformation, the encoding consisting in:
dividing each picture into blocks of picture elements, each block being represented by a block of brightness values, a block of blue colour difference values and a block of red colour difference values;
applying a two-dimensional transformation to each block of values in order to obtain a block of transformation coefficients of the block of values concerned;
transmitting, for each block of values, either the value of transformation coefficients of the block, or the difference in value of these transformation coefficients, with respect to the value of the transformation coefficients of a similar block in the picture preceding the picture being encoded, in order to minimize the quantity of information to be transmitted for the block in question, the encoding being called intra-picture or inter-picture respectively;
furthermore consisting in multiplying the transformation coefficients and the differences of transformation coefficients, before they are transmitted, by a coefficient called the weighting coefficients, favouring the low spatial frequencies of the pictures; and also in multiplying them by a coefficients called the quantification coefficient which is variable as a function of the quantity of information to be transmitted;
the decoding consisting for each block in:
multiplying each transmitted value of transformation coefficient or each transmitted value of transformation coefficient difference, by a coefficient equal to the inverse of the weighting coefficient used for the encoding and by a coefficient equal to the inverse of the quantification coefficient used for the encoding;
adding, to the value of each difference of transformation coefficients, the value of a transformation coefficient, similar to the coefficient in question in a similar block to the block in question and belonging to the picture preceding the picture being decoded;
applying to each transformation coefficient a transformation which is the inverse of the transformation applied for the encoding, in order to obtain a block of values representing a portion of the decoded picture;
is characterized in that it furthermore consists in regulating the data rate of the transmitted information, in storing in a same buffer memory, the information to be transmitted corresponding to the 3 types of blocks of values, and in multiplying the transformation coefficients and the differences of transformation coefficients of the 3 types of blocks by a same variable weighting coefficient and by a same variable quantification coefficient, except for the application of a constant multiplication factor.